The present invention relates to wide-band frequency modulation (FM) television receivers and, more particularly, to a FM color television receiver which includes a novel tracking filter circuit for improved noise threshold performance and minimal subjective color distortion.
Wide-band FM receivers are known in the art. Such receivers are used in frequency division multiplexed (FDM) multichannel telephony and in FM television systems. Recent wide-spread broadcasting of frequency-modulated television signals by communications satellites has created a demand for high performance FM television receivers for use in low-cost earth stations for private homes. Because such home earth stations are generally equipped with relatively small dish antennae, the receivers used in such earth stations must be capable of providing acceptable quality picture and sound despite the relatively weak signals that are obtained from the small antennae.
In general, the signal-to-noise ratio (SNR) of an FM receiver is directly proportional to the carrier-to-noise ratio (CNR), provided that the CNR is above a certain threshold value. Below that threshold, the SNR of the receiver falls off very rapidly with decreasing CNR, and in the case of a television receiver, the subjective quality of the picture and sound quickly degrades with diminishing SNR until it becomes no longer acceptable to the viewer. In a conventional FM receiver, the noise threshold typically occurs when the CNR is approximately 10 dB.
Several approaches are known for extending the noise threshold of an FM receiver to lower values of CNR. In general, each of these approaches achieves threshold extension by reducing the effective predetection bandwidth to less than the normal IF bandwidth of the receiver. In one such approach, which is commonly referred to as FM feedback (FMFB), the IF signal is passed through a fixed narrowband filter, and a discriminator in conjunction with a voltage-controlled local oscillator are used to keep the IF signal centered within the passband of the filter by automatic frequency control action. A description of the FMFB technique may be found in U.S. Pat. No. 2,075,503.
Another approach for extending the noise threshold of an FM receiver uses a phase-lock loop or phase-lock oscillator whereby the noise reduction filtering is obtained at baseband. Descriptions of this technique and a variation thereof can be found in articles entitled "Design and Performance of Phase-Locked Circuits Capable of Near Optimum Performance Over a Wide Range of Input Signal and Noise Levels", by R. M. Jaffee and E. Rechtin, IRE Transactions on Information Theory, Vol. IT-1, pp. 66-76, March, 1955, and "Synchronized Oscillators as F-M Receiver Limiters", by C. W. Carnahan and H. P. Kalmus, Electronics, August, 1944.
A third approach for extending the noise threshold in a FM receiver, which is described in U.S. Pat. No. 2,976,408, uses a closed-loop tracking filter inserted ahead of the demodulator of an otherwise conventional FM receiver. The tracking filter typically includes a voltage-tunable, bandpass filter (steerable filter) having a passband that is narrower than the normal IF bandwidth and a feedback control loop for keeping the passband of the steerable filter approximately centered on the instantaneous frequency of the IF signal. The control loop typically senses the phase shift of the steerable filter and uses the sensed phase shift to correct the tuning of the filter to track the IF signal. The output provided by the tracking filter is demodulated by a conventional FM demodulator.
For TV earth station applications the tracking filter approach to threshold extension is highly advantageous in that it inherently has a simpler control loop than either the FMFB or phase-locked loop approaches and thus minimizes control-loop delay time, which is an important parameter when high baseband frequencies are encountered such as those contained in a color TV signal. Another advantage results from the inherent configuration of the tracking filter approach whereby it can be added to a conventional FM receiver thereby upgrading it to an FM threshold extension receiver. Moreover, the tracking filter can be easily bypassed where the incoming signal is strong enough such that threshold extension is not required.
However, threshold extension techniques, including the tracking filter approach, have drawbacks when used in receivers for color television signals owing to a tendency of the threshold extension circuits to exhibit transmission nonlinearities when the control loop parameters are selected for optimal noise threshold performance. For the tracking filter approach, the control loop parameters include loop gain and the bandwidth of the steerable filter. Such nonlinearities can produce significant degradation of the subjective quality of the colors of a television picture. In this regard, it is noted that a typical television viewer is less tolerant of noise or distortion in the colors of a television picture than he or she is of noise or distortion in the picture itself. Consequently, design compromises must be made to the bandwidth and loop gain in order to reduce the subjective color distortion to an acceptable level. Since the effectiveness of the tracking filter circuit for threshold extension depends on the optimum bandwidth of the steerable filter and the accuracy with which it tracks the IF signal, such design compromises severely limit the noise threshold performance of the tracking filter circuit.
One known attempt at improving the performance of a tracking filter circuit in a FM color television receiver is to use a double-pole steerable filter, which provides a "flatter" passband and permits the elimination of a phase shifter in the tracking filter circuit. The use of such a steerable filter is described in an article entitled "An Advanced Threshold Extension Tracking Filter for Satellite Video Reception", by W. M. Rogers, Proceeding of the 1981 I.E.E.E. National Telecommunications Conference, Nov. 29-Dec. 3, 1981, pp. E6.5.1-E6.5.7. Although the use of a double-pole steerable filter in the tracking filter circuit of a FM television receiver does improve the noise threshold performance of the receiver to a degree, it is not entirely satisfactory in that such improvement in noise threshold performance is achieved at the expense of subjective color distortion in the television picture.
U.S. Pat. No. 3,792,357, having a common assignee and a common inventor with the present application, describes a technique for improving the performance of an FDM/FM multichannel telephony receiver having a threshold-extension tracking filter circuit by adding a loop filter in the feedback control loop of the tracking filter circuit. The loop filter used has characteristics that substantially match those of the pre-emphasis filter in the modulator of the transmitter. While the addition of a pre-emphasis loop filter to the tracking filter circuit substantially improves the performance of the FDM/FM receiver, the technique does not produce expected results when applied to an FM television receiver, because it cannot provide significant improvement in noise threshold performance without introducing subjective color distortion in the television picture.
Accordingly, a need exists for a FM television receiver having a tracking filter circuit that provides improved noise threshold performance without introducing unacceptable subjective color distortion.